The present invention relates to a photomultiplier tube having a narrow band spectral response and particularly to a photomultiplier tube having an ultraviolet spectral response (responsivity) within the wavelength range of about 300 to less than 400 nanometers (nm). This type of tube is known as a "solar blind" tube because of its sensitivity in the ultraviolet portion and insensitivity to the visible portion of the electromagnetic spectrum.
U.S. Pat. No. 4,196,257, issued to Engstrom et al. on Apr. 1, 1980, discloses a bi-alkali telluride photocathode for a "solar blind" photomultiplier tube which has high responsivity at wavelengths shorter than 290 nm. However, the responsivity of the bi-alkali telluride photoemissive cathode within the wavelength range of 300 to 400 nm is less than 1 milliampere per watt (mA/w). The bi-alkali telluride photocathode is formed on the interior surface of a transparent glass faceplate which transmits light in the ultraviolet, visible and near infrared portions of the electromagnetic spectrum.
It is known in the art that photomultiplier tubes having conventional alkali and bi-alkali antimonide photoemissive cathodes formed on transparent faceplates, such as ultraviolet grade sapphire or 9741 glass, or the equivalent, intrinsically have high responsivity in the near ultraviolet and visible portions of the electromagnetic spectrum. A typical spectral response curve for a potassium-cesium-antimonide photoemissive cathode formed on the transparent glass faceplate, which transmits in the ultraviolet as well as the visible portion of the spectrum, is shown in FIG. 1. One technique, known in the art, for limiting the repsonsivity of such tubes to the ultraviolet portion of the spectrum is to interpose an ultraviolet transmitting filter between the radiation source and the faceplate of the tube. The ultraviolet transmitting filter transmits predominantly the ultraviolet portion of the spectrum and substantially blocks the visible portion of the spectrum, thereby creating a "solar blind" tube from a conventional photomultiplier tube. The drawback of such a tube and filter combination is that multiple reflections occur from the surfaces of the filter and the tube. As shown in FIG. 2, the incident radiation, represented by incident ray I, is reflected from the front and back surfaces of the ultraviolet filter and also from the front and back surfaces of the tube faceplate before impinging on the photoemissive cathode formed on the interior surface of the tube faceplate. The four reflections, r.sub.1, r.sub.2, r.sub.3 and r.sub.4, significantly reduce the radiation transmitted to the photoemissive cathode and, therefore, reduce the radiation available to the photoemissive cathode for producing photoelectrons. Additionally, means must be provided to position the filter between the radiation source and the faceplate and to prevent stray radiation from impinging on the tube faceplate. Cementing the filter to the faceplate is generally avoided, because the adhesive further attenuates the ultraviolet radiation into the faceplate by absorption in the adhesive. Mechanical structures for attaching the filter between the tube and the radiation source and for preventing light-leaks therebetween are costly.
A filter and tube combination, for use with a television picture tube, is disclosed in U.S. Pat. No. 3,143,683, issued to Duncan et al. on Aug. 4, 1964. The filter is generally described, in one embodiment, as comprising an implosion plate or window which is either spaced from or laminated to the faceplate of the television tube. The patent also discloses, in another embodiment, that the filter may comprise the faceplate of the television tube. The purpose of the filter, in both embodiments, is to help equalize the intensity of the phosphor colors and to absorb ambient light at about 580 nm, the portion of the spectrum near where the eye is most sensitive. The ambient light, if reflected to the viewer, cuts down on color definition and contrast. However, uniform absorption throughout the visible spectrum is not beneficial in this application since this would also absorb the blue-, green- and red-emitted phosphor colors. Since it is known that the phosphor elements emit very little light in the portions of the visible spectrum between the colors, the filter is designed to selectively absorb at 580 nm where the deleterious effect of the ambient light can be reduced without decreasing the brightness of the television picture. The patent discloses that at the wavelength of greatest absorption (580 nm), the filter still transmits 45.2% of the radiant energy. At wavelengths on either side of 580 nm, the radiant energy transmittance of the filter increases to enhance the transmission of the blue, green and red light from the phosphor elements.
The fact that the Duncan et al. patent utilizes a filter faceplate for a television picture tube to enhance color transmission through the faceplate and to selectively absorb ambient light that would otherwise be reflected to the viewer and would reduce color definition and contrast does not suggest the use of a filter faceplate for a "solar blind" photomultiplier tube. Such a "solar blind" photomultiplier tube requires a filter which blocks the entire visible spectrum and passes only the ultraviolet spectrum. However, such a filter poses problems unique to photomultiplier tubes in that visible light is used during the formation of the photoemissive cathode. U.S. Pat. No. 2,676,282, issued to Polkosky on Apr. 20, 1954, discloses a method of depositing a conductive metal film on the interior surface of a faceplate by monitoring the transmission of light through the film. The use of an incandescent tungsten light source for monitoring photocurrent during the formation of a photoemissive cathode is described in U.S. Pat. No. 4,306,188, issued to Ibaugh on Dec. 15, 1981. The methods described in the Polkosky and Ibaugh patents would seem to be negated if a filter, which blocks the visible spectrum, were used as a faceplate for a photomultiplier tube.
Accordingly, the need exists for a cost effective way of producing a "solar blind" photomultiplier tube having high responsivity in the wavelength range of about 300 to less than 400 nm.